This invention relates to radiation detection systems and more particularly to radiation detection systems having a wide field of view and narrowband (or sharp cut-on or cutoff) detection requirements.
In many applications it is desirable or even necessary to provide a detection system which looks into a very large field of view. This may be because of an inherent desire to have large fields of view or it may be so that the received signals are relatively independent of the angular position of the detectors. In such applications the usual solution is to provide a bare detector staring into the world. A flat plate detector looks into two .pi. steradians.
In many of these systems it is also a desired characteristic to minimize the band of frequencies incident on the detector, usually to enhance signal to noise ratio. To provide minimum bandwidth an interference filter is conventionally placed over the detector. This, however, can create problems in wide field of view systems. For example, when an interference filter is used in conjunction with a detector for wide field of view systems, the filtering characteristics of the filter change depending on the angle of incidence of radiation incident on the filter. When the angle of incidence of the radiation impinging on the filter changes from the normal, the wave amplitude at the boundary of the interfering wave changes, thereby changing the filtering characteristics of the filter, such as a shift in bandpass, or a shift of the bandedge.
The transmission qualities of the filter are thus shifted as a function of the impingement angle, the angle between that of the incident radiation and a line normal to the detector surface. The peak transmission of the filter is shifted as a function of the impingement angle, and the total transmission is decreased as the impingement angle grows larger. Therefore, the use of interference filters in a wide field of view system becomes a problem. The problem is further increased when the system is to operate over a varying temperature range, since filtering properties of the filter also change with temperature.
In order to overcome these problems, particularly the use of interference filters in wide field of view systems, several solutions have been used in the past. The simplest solution is to provide more than one flat detector. This is shown in FIGS. 1A and 1B of the drawings. In FIG. 1A two detectors 10 and 12 are provided; and in FIG. 1B three detectors 14, 16, and 18 are provided. For this solution it is seen that as one wishes to increase the field of view of the system several detector surfaces are required. Of course, this is both expensive and complex. Furthermore, these sytems have the problem that seams between the detectors must be made exceedingly small to avoid field gaps.
Another solution is to use a curved detector. However, this is exceedingly difficult to provide from a manufacturing viewpoint, as well as expensive.